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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 191401

Area Forecast Discussion
National Weather Service Honolulu HI
401 AM HST Tue Mar 19 2019

A high pressure system will drift eastward today passing just
north of the Hawaiian Islands, keeping moderate trade winds in
the forecast. Dry stable air aloft will limit shower activity for
much of the week. On Wednesday, a trough associated with a cold
front approaching the islands from the north will weaken the ridge
north of the islands producing light winds. High pressure will
build back in over the region from Thursday onward with increasing
trade wind speeds. A cold front will bring clouds and showers to
the islands on Saturday and Sunday. Strong trade winds will
arrive with the frontal passage.


This mornings satellite imagery shows an extensive area of stable
stratocumulus clouds that developed overnight around the Hawaiian
Islands. These clouds will decrease in coverage through the
morning hours and precipitation coverage will remain limited.
Morning soundings from Lihue and Hilo are showing a strong trade
wind subsidence inversion aloft around 5000 to 7000 feet capping
vertical cloud development today.

A surface high north of the state will drift into the East
Pacific today keeping a dry moderate trade wind pattern in the
forecast for today. As the high drifts east, a trough with an
associated cold front moves into the Central Pacific basin
weakening the ridge north of the islands on Wednesday. Surface
pressure gradients will weaken in response to this approaching
trough on Wednesday with onshore sea breezes forecast to develop
over leeward coasts and slopes by Wednesday afternoon.

On Thursday, another surface high builds in north of the state
with a return to the trade wind flow and isolated to scattered
showers over windward and mauka areas. Trade wind speeds will
increase over the next two days, becoming breezy on Friday and
locally windy on Saturday.

On Saturday, the cold front arrives over the northwestern islands
bringing in clouds and elevated showers to the islands of Kauai
and Oahu. This front will slowly spread south down the island
chain on Saturday night, reaching the Big Island on Sunday. The
arrival timing with this front will be tricky as the movement
towards the islands will be driven by the low level winds blowing
out of the high center behind the front. We had to delay the
arrival timing of this front by roughly 12 hours this morning due
to updated model guidance showing a slower and weaker front. Both
the American (GFS) model and the European (ECMWF) model continue
to show run to run timing differences keeping our confidence on
start and stop times for this event squarely in the fair category.
That said, stronger trade winds arriving with this system will
enhance clouds and showers along windward and mountain areas.
Some of the stronger showers will drift over leeward areas. For
outdoor activities this weekend, plan for breezy to windy
conditions with periods of rain showers.

Weather conditions will improve after frontal passage. Some clouds
and scattered showers will linger over windward and mauka areas
with slightly higher shower activity forecast during the overnight
and early morning hours. Moderate trade winds will continue
through the first half of next week.


Today, surface high pressure north of the region will generate
light to moderate easterly flow, generally veering to east
southeasterly by this afternoon. Limited shallow moisture should
allow for only ISOL -SHRA along the windward coasts and slopes.
Otherwise, VFR conditions prevail at the local airports.


Moderate to fresh trades will continue today, then ease into the
light to locally moderate range tonight through Wednesday night as
a front passes by well to the north of the islands. A Small Craft
Advisory (SCA) remains in effect for most waters today due to
elevated seas associated with the current northwest swell, with
all marine zones expected to drop below SCA levels tonight as this
swell fades. A new northwest swell will build on Wednesday
however, and this will likely bring seas back up to SCA levels
across portions of the marine area exposed to this swell. The
trades will begin to re-strengthen Thursday as a new high builds
northwest of the state, reaching moderate to strong speeds Thursday
night through the weekend. The increasing trades in combination
with a series of overlapping northwest swells will likely require
SCAs for a good chunk of the marine area Thursday through the

Surf along north and west facing shores continues to slowly
decline early this morning, but the latest buoy observations
continue to support advisory level surf along most north and west
facing shores. As a result, the High Surf Warning (HSW) for north
and west facing shores of Niihau, Kauai, Oahu, Molokai and north
facing shores of Maui has been cancelled and replaced with a High
Surf Advisory (HSA) through 6 PM this evening. The swell has
dropped below HSW and HSA levels along north facing shores of the
Big Island, so the HSW has been cancelled here. Surf along west
facing shores of the Big Island has dropped below HSA levels, so
the HSA has been cancelled here. Surf is expected to drop below
advisory levels along north and west facing shores of all islands
early this evening. A new northwest swell will be hot on the
heels of the current fading northwest swell however, and this will
likely bring surf back up to advisory levels across north and
west facing shores exposed to this swell once again on Wednesday.
A series of overlapping northwest swells will keep surf elevated
along north and west facing shores through the remainder of the
week and over the weekend, with surf remaining near or at advisory
levels Wednesday night through Saturday.

Surf along east facing shores will remain small through midweek,
then rise Friday into the weekend as strong trades return locally
and upstream. Surf along south facing shores will remain small
through the week with mainly background southerly pulses moving


High Surf Advisory until 6 PM HST this evening for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward

Small Craft Advisory until 6 PM HST this evening for Kauai
Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-
Kauai Channel-Oahu Windward Waters-Kaiwi Channel-Maui County
Windward Waters-Alenuihaha Channel-Big Island Windward Waters-
Big Island Leeward Waters-Big Island Southeast Waters.




Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office

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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
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